Breast cancer is the second leading cause of cancer death in American women and family history is known to be a significant risk factor for the development of this disease., Recently, genetic linkage analysis has identified chromosome 17q2l as the locale of a gene which confers susceptibility to early-onset breast cancer. This study also adds support to previous evidence that the trait is transmitted in an autosomal dominant fashion and may involve loss or inactivation of a tumor suppressor gene. In collaboration with the group responsible for linkage analysis (Dr. M.C. King and colleagues), we plan to further localize and ultimately clone this gene. Our goal is to identify at least twenty-five families with a high incidence of breast cancer (excluding Li-Fraumeni families) with an average age at diagnosis less than 45 years. Once these families have been identified, samples will be collected for DNA preparation and linkage studies performed using mapping probes being prepared from 17q2l-q23 by Dr. King's group and the Genetic Marker Core of the Michigan Genome Center. Peripheral blood lymphocytes will be collected from living family members and we provide evidence that formalin-fixed, paraffin-embedded tissue is an excellent source of DNA for linkage analysis from deceased family members. One affected family member will also undergo karyotyping in an attempt to identify balanced translocations or other visible chromosomal abnormalities which may involve the breast cancer locus. Once the breast cancer region has been narrowed to 1-4 cM from its current estimated size of 20cM, a physical map will be constructed using YAC cloning, YAC walking and contig construction. The contigs encompassed by the most tightly linked markers will be searched for potential transcripts with several strategies including cDNA screening, CpG island identification and exon trapping. Potential candidate sequences will be analyzed for differences in expression between normal and malignant breast tissue and for the presence of mutations in affected individuals. Successful completion of this study will provide the foundation for work which promises to be pivotal in our understanding of the development of breast cancer.